Low-nickel valve steel

ABSTRACT

THIS IS AN AUSTENITIC STAINLESS STEEL, PARTICULARLY SUITED FOR USE IN THE MANUFACTURE OF INTERNAL COMBUSTION ENGINE VALVES CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT, .45 TO .57 CARBON, 7 TO 12 MANAGANESE, .25 MAX. SILICON, 17 TO 22 AND PREFERABLY 18 TO 21.5 CHROMIUM, 1 TO 4 NICKEL, 40 TO .60 NITROGEN, TOTAL ADDITIONAL CARBIDE-FORMING ELEMENT CONTENT OF 1.5 TO 4.5 WITH VANADIUM UP TO .45, AND THE BALANCE IRON. PREFERABLY, SILICON IS .15% MAX. THE STEEL IS CHARACTERIZED BY GOOD ELEVATED-TEMPERATURE STRENGTH AND LEAD-OXIDE CORROSION RESISTANCE.

Aug. 1, 1972 J DULIS ETAL 3,681,058

LOW-NICKEL VALVE STEEL Filed Nov. 12, 1969 2 Sheets-Sheet l 3.00 I I I Q U s (r) g 2.00 \l i LIMIT OF ACCEPTABLE g comos/o/v RES/STANCE u .L

/.00 I I I EFFECT OF VA/VAD/UM CONTENT ON, LEAD OXIDE CORROSION RES/STANCE INVENTORS EDWARD J. OUL/S, AUGUST KASAK 0 WILLIAM STA KO Attorney- United States Patent 3,681,058 LOW-NICKEL VALVE STEEL Edward J. Dulis, Mount Lebanon Township, Allegheny County, August Kasak, Upper St. Clair Township, Allegheny County, and William Stasko, Munhall Borough, Pa., assignors to Crucible Inc., Pittsburgh, Pa.

Filed Nov. 12, 1569, Ser. No. 875,851 Int. Cl. C22c 39/20 US. Cl. 75-128 A 2 Claims ABSTRACT OF THE DISCLOSURE This is an austenitic stainless steel, particularly suited for use in the manufacture of internal combustion engine valves consisting essentially of, in weight percent, .45 to .57 carbon, 7 to 12 manganese, .25 max. silicon, 17 to 22 and preferably 18 to 21.5 chromium, 1 to 4 nickel, .40 to .60 nitrogen, total .additional carbide-forming element content of 1.5 to 4.5 with vanadium up to .45, and the balance iron. Preferably, silicon is .15% max. The steel is characterized by good elevated-temperature strength and lead-oxide corrosion resistance.

Steels for use in the manufacture of automotive internal combustion exhaust valves are normally austenitic, because of the significant strength advantage provided by austenitic steels over martensitic or ferritic steels at the high service temperatures encountered by this material when used in the manufacture of valves of this type. Heretofore, the service temperatures were on the order of about 1200 to 1350 F. Recently, however, higher engine-operating temperatures have become more common, and now the material used in valve manufacture must withstand engine temperatures approaching about 1500 F. Consequently, conventional materials used for the manufacture of valves typically lack the necessary high-temperature strength for use at these higher engine temperatures. There are some nickel-, iron-, and cobaltbase superalloys that possess the necessary strength at these high temperatures, principally because of their highalloy content, but most of these alloys generally lack the necessary corrosion resistance, specifically resistance to lead-oxide corrosion encountered in exhaust gas environments. Furthermore, these superalloys are extremely expensive as a result of their high-alloy content. Moreover, these superalloys, even if they do not have a nickel base, nevertheless contain large amounts of nickel for purposes of promoting strength and austenite stability. The element nickel in recent years characteristically experiences fluctuations with respect to availability.

It is accordingly the primary object of this invention to provide an austenitic stainless steel suitable for use in the manufacture of internal combustion engine valves required to operate at engine temperatures of 1500 F. and higher.

Yet another object of the invention is to provide an austenitic stainless steel for internal combustion engine valves operating at high temperatures characterized by a good combination of elevated temperature strength or heat resistance and lead-oxide corrosion resistance.

Another more specific object of the invention is to provide an austenitic stainless steel for the manufacture of internal combustion engine valves subjected to high temperature service conditions, the steel being characterized by good strength and heat resistance at these high temperatures, while having a relatively low alloy content, particularly with respect to nickel which need be present in an amount within the range of 1 to 4% These and other objects of the invention as well as a complete understanding thereof may be obtained from 3,681,058 Patented Aug. 1, 1972 ice the following description, specific examples and drawings, in which:

FIG. 1 is a graph showing the criticality of an upper limit for vanadium in the alloy of the invention for the purpose of providing improved lead-oxide corrosion resistance; and

FIG. 2 are photographs of samples of material with varying vanadium contents to show the effect of vanadium with respect to the scaling characteristics of the stainless steel of the invention.

The austenitic stainless steel of the invention, as pointed out above, must have good elevated temperature strength to withstand the high-temperature service conditions, which may be on the order of about 1500 F. Consequently, the composition of the alloy must be balanced to render it stable and fully austenitic. In addition to the required elevated-temperature strength, the alloy must possess good corrosion resistance in the lead-oxide type environments to which it will be subjected, and further the alloy must possess good surface oxidation characteristics in that the oxide formed at the high service temperatures must be line and tightly adhering rather than being of the large, scaly type oxide. The latter type oxide formation render the valves susceptible to hot spots, which in turn cause burning and premature valve failure.

The objects of the invention as outlined hereinabo-ve are achieved by providing an austenitic stainless steel consisting essentially of, in weight percent, .1 to .75 and preferably .45 to .75 carbon, 4 to 12 and preferably 7 to 12 manganese, .25 max. silicon and preferably .15 max. silicon, 17 to 22 chromium and preferably 18 to 21.5 chromium, 1 to 4 nickel, .1 to .6 and preferably .40 to .60 nitrogen, total additional carbide-forming element content of 1.5 to 4.5 with vanadium up to .45, and the balance iron. The carbon plus nitrogen should be sufficient to give an essentially complete austenite matrix.

Preferably with the above composition specific carbide promoters may be present in amounts of up to .45% vanadium, preferably .3 to .45% vanadium, 1 to about 3% tungsten, .50 to about 1.5% columbium with the total content of these carbide-forming elements not exceeding about 4.5%. Molbydenum may be substituted for tungsten on an atomic weight basis. Examples of other strong carbide formers are zirconium, titanium, and tantalum.

Chromium, which is a ferrite promoter, is necessary within the above-recited ranges to provide the alloy with corrosion resistance and oxidation resistance; however, the chromium content if too high, will cause ferrite formation, which reduces the elevated-temperature strength of the alloy.

Manganese is an austenite promoter and in conjunction with nickel, which is also an austenite promoter, contributes with respect to the alloy balance in achieving the stable austenitic structure required for achieving hightemperature strength. Manganese, in conjunction with nickel, promotes corrosion resistance, as well as promoting the stretch resistance of the alloy at elevated temperature.

Nickel promotes the desired austenitic structure in the alloy and is critical within the above-recited range in achieving good stretch resistance at elevated temperature. Stretch resistance, as will be pointed out in greater detail hereinafter, determines the resistance of a valve made of the alloy to deformation at elevated temperature. Valve deformation during service at elevated temperature is a primary cause of valve failure at high operating temperature service conditions. Nickel within the range of 1 to 4% is necessary for this purpose; however, increased amounts of nickel above about 4% no longer significantly improve stretch resistance and add significantly to the cost of the alloy.

Vanadium, as well as other carbide-forming elements that may be used in the alloy, promotes high-temperature strength. In so doing, vanadium combines with the carbon in the alloy to form carbides, which in turn reduces the austenite stability of the alloy otherwise provided by the presence of carbon, which is an austenitepromoting element. Vanadium is also critical with respect to providing lead-oxide corrosion resistance. Specifically, as will be shown in detail hereinafter, the presence of vanadium above about .45 drastically reduces the leadoxide corrosion resistance of the alloy. In addition, in creased amounts of vanadium above this level also have an undesirable effect with respect to the surface oxide characteristics of the alloy.

Tungsten, which is also a carbide former, combines with carbon to form carbides that are stable at elevated service temperature conditions and thus improve the elevated-temperature strength of the alloy. Excess amounts of tungsten in combination with the total carbide forming element content will impair the austenite stability of the alloy.

Columbium like tungsten functions as a carbide former that enhances the elevated temperature strength of the alloy.

Carbon is an austenite former as well as a carbide former. In view of the carbide-forming element content of the alloy carbon plus nitrogen must be maintained within the recited range to provide suflicient carbon plus nitrogen to maintain an austenitic matrix and to combine with the carbide-forming elements to form carbides and/ or nitrides that at elevated temperature provide the required strengthening for the alloy.

Nitrogen, like carbon, is an austenite former and'must be maintained within the above-described range to provide alloy balance. In addition, as explained above, it with carbon combines with the carbide-forming elements to form nitrides and carbonitrides, which serve as strengtheners in the alloy.

Silicon must be maintained at the above-described maximums to achieve the desired lead-oxide corrosion resistance. Increased silicon above the above-recited maximums'will render valves made of the alloy susceptible to deterioration at high temperature by corrosion in the engine exhaust environment.

By way of specific examples and to demonstrate the criticality and advantages of an austenitic stainless steel in accordance. with the invention, a series of alloys was produced and tested, the compositions of which are listed in Table I.

TABLE I.-COMPOSITSION OF STEELS ture for a predetermined number of hours or until a certain amount of total deformation of the samples being tested is obtained. Steels that are more stretch resistant exhibit less total elongation in a specified test time and consequently may be considered as more heat resistant or more resistant to creep deformation at elevated temperatures. Table II presents the stretch test results for selected compositions of Table I. More specifically, the data presented in Table II shows the effect of nickel With respect to the stretch resistance in austenitic chromium-manganese-carbon-nitrogen steels of the type of the invention.

TABLE II.--STRETCH TEST RESULTS Heat Percent number 1 nickel 1,350 F.-18,000 p.s.i. 1,500 F.9,000 p.s.i.

1 Solution treated at 2,150 F. for 1 hour; water quenched; aged at 1,400" F. for 16 hours.

As may be seen from Table II, in the absence of nickel within the range for the alloy of the invention (Heat No. ID39 having 0.03% nickel) significant elongation after 100 hours of exposure is produced under both of the testing conditions reported in Table II. Significant improvement is achieved by increasing nickel within the range of 1 to 4% in accordance with the invention. However, no significant additional improvement is achieved by increasing nickel substantially above 4%, which is the upper limit in accordance with the invention. In fact, by significantly increasing nickel above the upper limit as provided for in the invention some deteriation in the stretch resistance of the material results. Also, of course, increased nickel is detrimental from both the cost and availability standpoints.

As explained hereinabove, during service automotive exhaust vales are exposed to the corrosive attack of the high-temperature combustion products of lead-containing fuels. Consequently, gogd corrosion resistance -'at high temperatures in lead-oxide-containing environments is required at high temperatures in lead-oxide-containing environments is required in steels for use in the manufacture of automotive internal combustion engine exhaust valves.

The corrosion resistance of a steel to lead oxide is evaluated by the lead-oxide test. In this laboratory test, specimens of the steel of approximately 0.5 in. diameter and approximately 0.5 in. length are machined, given a standard finish, measured, Weighed, and then placed in a mag- Weight percent 6 Mn 81 Cr Ni V W Cb N Selected alloys from the compositions listed in Table I nesia crucible together with 40 grams of lead oxide, were tested for stretch resistance at a temperature of placed in a furnace, and heated at 1675 F. for one hour. 1350 F. and 18,000 p.s.i., and at a temperature of 1500 The crucibles are then emptied. Specimens are cooled to F. and 9,000 p.s.i. Failure of exhaust valves in internal room temperature, descaled in molten caustic, cleaned, combustion engine applications at high operating temand weighed again. The weight loss per unit area of the peratures. can occur by creep deformation of the valves. specimen indicates the relative corrosion resistance of the The resistance to th1s, or creep resistance, of steels is steel of the specimen. The lower the weight loss in grams evaluated by the stretch test. This test consists of subjectper square inch, the greater is the corrosion resistance of ing test specimens of the steel to stress at a given temperathe steel. Results of lead-oxide corrosion tests performed TABLE III.RESULTS OF LEAD OXIDE CORROSION TEST Vanadium Weight ss percent g.

Heat number I As may be seen from the data reported in Table III and from the graph of FIG. 1 of the drawings, the vanadium content of the alloy of the invention is particularly critical with respect to maintaining an upper limit of about .45 to achieve the required leado-xide corrosion resistance in the steel. Specifically with respect to FIG. 1, the graph thereof shows that as vanadium is increased above about .45% the corrosion resistance, as signified by weight loss, drastically increases. Consequently, an upper limit for vanadium in the alloy of the invention is .45% for the purpose of maintaining acceptable lead-oxide corrosion resistance.

Prolonged exposure of exhaust valves at elevated temperature ultimately results in surface oxidation of the valves. The type or form of the oxide has a significant influence on the service life of the valve. For example, if the oxide is of the large particle, loosely adherent type it can adhere to the valve seat, which results in localized overheating and ultimate failure of the valve. Consequently, it is preferred that the oxide form be tightly adherent or if loose be in the form of fine particles, rather than oxide particles that are large and loosely adherent.

Oxidation tests were performed on selected compositions listed in Table I. These tests were conducted by exposing specimens in an air atmosphere at 1800 F. for five 20-hour periods for a total exposure of 100 hours. Test specimens having a 0.5 in. diameter x 1.25 in. long were machined, ground over their entire surface, measured, and degreased. The test specimens were then placed in a porcelain crucible, exposed for 20 hours and cooled to room temperature. This procedure was repeated until IOU-hour exposure was accumulated.

The results of the oxidation tests are shown in FIG. 2, which presents photographs of the specimens after testing. The specimens each were of a composition of Table I having various vanadium contents, as indicated on FIG. 2. As may be seen from the photographs on FIG. 2 when vanadium is significantly increased over the upper limit of .45%, e.g. 0.58% vanadium and higher, the oxide formed during the oxidation tests is of the undesirable, large-flake type. In contrast, however, by maintaining vanadium within the limts of the invention the oxide is a fine, loose type which is easily dispersed by the exhaust gases and will not adhere to the valve seat and cause localized overheating and valve failure. It may be seen, therefore, that the upper limit for vanadium is significant in the alloy of the invention for the purpose of controlling the type or character of the oxide formed during hightemperature service and thus promoting prolonged valve service life.

While we have shown and described herein certain embodiments of our invention, we intend to cover as well any change or modification therein which may be made without departing from the scope and spirit of the appended claims.

We claim:

1. An austenite stainless steel characterized by good elevated-temperature strength and lead-oxide corrosion resistance consisting essentially of, in weight percent, .45 to .57 carbon, 7 to 12 maganese, .25 max. silicon, 17 to 22 chromium, 1 to 4 nickel, .40 to .60 nitrogen, up to .45 vanadium, 1 to about 3 tungsten, .50 to about 1.5 columbium with the total content of vanadium, tungsten and columbium not exceeding about 4.5%, and the balance iron.

2. The steel of claim 1 having chromium of 18 to 21.5% and silicon of .15 max.

References Cited UNITED STATES PATENTS 2,745,777 5/1956 Clarke -128 A 3,310,396 3/1967 Hochmann 75'128 G 2,848,323 8/1958 Harris 75-128 N 3,165,400 1/1965 Roy 75128 N 2,799,577 7/1957 Schernpp 75-128 A 2,880,085 3/1959 Kirkby 75-128 G 3,152,934 10/1964 Lula 75--128 N 3,235,417 2/1966 Roy 75-128 A 3,401,036 9/1968 Dulis 75-128 N HYLAND BIZOT, Primary Examiner US. Cl. X.R.

75--128 N, 12 8 G, 128 V, 128 W 

